Most presidents of the United States presumably understand that the Constitution limits their power. Nevertheless, when America goes to war, many presidents have ignored constitutional constraints, disregarding Congress and deceiving the public.

In Presidents of War, historian Michael Beschloss builds upon an impressive mountain of research to make the case that from 1807 until today, presidents have routinely abused their authority when they assume the role of warrior-in-chief.

Presidents of War, by Michael Beschloss

(Crown)

One early and often overlooked example is the war between the United States and Mexico, which President James K. Polk began in 1846 following the annexation of Texas. This was fundamentally a war of conquest that added a million square miles, including California and New Mexico, to American territory.

Beschloss asks whether this expansion could have occurred peacefully, with war waged only as a last resort. He writes that "Polk's compulsive secrecy" and his lying to Congress about his war aims "violated the open, democratic tradition that the founders had tried to encourage."

Little more than a decade later, with the Civil War underway, writes Beschloss, Abraham Lincoln "transformed himself into the most powerful chief executive that Americans had ever seen — and by means that, as Lincoln confessed, were not 'strictly legal.'" Lincoln's argument for expanding his power was basically that the end justifies the means, an inherently dangerous concept that other wartime presidents have also found useful.

As they have expanded their wartime powers, presidents have sometimes been aided by the news media. In 1898, William McKinley, backed by screaming headlines provided by powerful newspaper publishers, used the Spanish-American War to define the United States as an emerging world power. McKinley, notes Beschloss, "transformed himself into an apostle of empire with the zealotry of a convert, but he showed little understanding of how the nation's new imperial role might ultimately compromise its image before the world — and its original revolutionary conception of itself."

Michael Beschloss.

(Stephen Voss/Crown)

Such zealotry and abandonment of principles can transform a presidency. Woodrow Wilson had spoken of being "too proud to fight," but once he decided to lead America into World War I, he embraced measures such as the Espionage Act of 1917. This sweeping law, as Beschloss observes, gave him "extraordinary control over published material and free speech."

Presidents have not been shy about relying on and expanding the wartime powers their predecessors exercised. Beschloss notes that Harry Truman, in the Korean War, was the first president "to engage the country in a major foreign conflict ... without bothering to ask Congress for a war declaration." Truman, writes Beschloss, was reluctant to deal with Congress about Korea and said he admired James K. Polk because Polk "regularly told Congress to go to hell on foreign policy matters."

Truman and later Lyndon Johnson and others, observes Beschloss, "proved that the Constitution's demand for congressional war declarations could be ignored without serious penalty." The American public usually tolerates this, at least for a while, but then — especially if the conflict drags on — may become restive, and the president's political fortunes may suffer.

President Lyndon B. Johnson speaks with Gen. William Westmoreland during a visit to Cam Ranh Bay in South Vietnam on Dec. 23, 1967.

(Yoichi Okamoto/The New York Times)

The speed and reach of modern weaponry necessitate giving presidents today some leeway to repel or retaliate against an attack. But other than that, the words of the Constitution's Article 1, Section 8 are clear: "The Congress shall have power...to declare war."

Presidential authority was never meant to be unchecked, even in wartime. Beschloss presents example after example of presidents fighting not so much against Congress as against the Constitution in their efforts to expand their powers as commander-in-chief.

He discusses presidents and issues with clarity that will appeal to the non-expert as well as students of the presidency and American wars. The cases that he presents so convincingly should remind Congress and the public that even when the country is at war, a president's power does not supersede the Constitution. That is a principle that too many chief executives have ignored.